This invention relates to redundancy reduction systems and, more particularly, to a redundancy reduction system for use with video signals.
Video signals of the type generated in connection with a video telephone service tend to contain a large amount of redundancy on a frame-to-frame basis. In U.S. Pat. No. 3,553,361, issued to F. W. Mounts on Jan. 5. 1971, a redundancy reduction system is described in which a video amplitude sample is transmitted to a receiving location only if that sample differs significantly from a corresponding stored sample having the same position in a previous video frame. This type of redundancy reduction system is known to those skilled in the art as a conditional replenishment video system. In order to enable the receiver to insert the sample into the proper location in a receiver frame memory, a position or address code is transmitted along with the amplitude value for a selected sample, followed by the amplitude values for the samples in succeeding address locations providing that these succeeding samples are also selected for transmission. Transmission of a group of samples is terminated by transmitting a flag word whose value is distinguishable from all amplitude values. The number of bits necessary to identify the position of an amplitude sample within the video frame is reduced to the number of bits which are necessary to locate a sample within a single video line. Synchronization between transmitting and receiving locations is maintained by transmitting at the end of each line and at the end of each frame sync words whose values are distinguishable from all address codes. Thus, a sync word equal in size to an address word is transmitted for each line of the video signal.